Rubber having high tensile strength and good abrasion resistance are required for a high-performance tire. A high-molecular-weight rubber having incorporated therein a large amount of a reinforcing agent is used for such rubber. However, a high-molecular-weight rubber has a high Mooney viscosity and poor processability, and therefore, a reinforcing agent is difficult to uniformly disperse therein by kneading. For kneading the rubber with a large amount of a reinforcing agent, a high power and a substantially long time are required. Thus, compatibility of good processability with high tensile strength and good abrasion resistance is eagerly desired.
In Japanese Examined Patent Publication (hereinafter abbreviated to as “JP-B”) No. H5-74614, a rubber composition having high tensile strength and good abrasion resistance is described, which comprises a low-molecular-weight aromatic vinyl-diene copolymer, obtained by polymerization carried out using an organolithium compound in a hydrocarbon solvent, and a styrene-butadiene copolymer rubber. In Japanese Unexamined Patent Publication (hereinafter abbreviated to as “JP-A”) No. H6-65418, a rubber composition having high tensile strength and good abrasion resistance is described, which comprises a modified low-molecular-weight aromatic vinyl-diene copolymer which has been obtained by anion living polymerization and further modification with an N-substituted amino compound, and a styrene-butadiene copolymer rubber.
In JP-A H6-200075, a rubber composition having reduced heat-build up and good abrasion resistance is described, which comprises natural rubber and/or polyisoprene rubber, a specific carbon black, and liquid polybutadiene, liquid polyisoprene or liquid styrene-butadiene copolymer rubber.
The above-described rubber compositions are still insufficient in compatibility of good processability with high tensile strength and good abrasion resistance.
In JP-A H6-278410, a tire having a cap tread comprised of a rubber composition comprising a raw material rubber having incorporated therewith a low-molecular-weight diene polymer is described. This tire is said as exhibiting a reduced change of tire tread with time and having good abrasion resistance. However, its abrasion resistance is still insufficient.
In JP-B H6-86500, a process for producing a conjugated diolefin polymer is described, which has an average vinyl bond content of at least 50% by weight in the conjugated diolefin unit portion, contains 5% to 30% by weight of a fraction having a molecular weight of smaller than 100,000 and having a vinyl bond content of smaller than 50% by weight, has a Mooney viscosity (ML1+4,100° C.) in the range of 30 to 100, and contains at least 30% by weight of branched polymers. This conjugated diene polymer is produced by the steps of polymerizing a conjugated diolefin and an aromatic vinyl compound using an organolithium initiator in the presence of a Lewis base in a hydrocarbon solvent; adding a deactivator for deactivating the active terminals of polymer chains after the completion of polymerization; and further additional amounts of the conjugated diolefin and the aromatic vinyl compounds, the hydrocarbon solvent and the Lewis base were added to conduct polymerization. This production process has problems such that the polymerization is very difficult to carry out under thoroughly controlled conditions, and the production efficiency is low. Further, although the conjugated diolefin polymer is said as having high tensile strength and good abrasion resistance, the tensile strength and the abrasion resistance have not been enhanced to the sufficient extent.